High surface pressure resistant steel parts and methods of producing same

ABSTRACT

High surface pressure resistant steel parts and their producing methods are disclosed. These steel parts are useful as gears, cams, bearings and similar high-strength compact steel articles which are required to have wear resistance and strength to withstand fatigue in rolling or rolling-slipping applications. In a steel part formed according to the invention, a fine nitride and/or carbonitride having at least an average grain size of 0.3 μm or less is dispersed in the contact surface structure; a multi phase structure composed of martensite, which is divided into extremely fine pieces, forming a disordered shape, by the nitride and/or carbonitride, is formed; and a carbide having a grain size of 3 μm or less is dispersed to increase the hardness of the surface. Such a steel part is produced by carrying out carbonitriding or carburization/carbonitriding so as to precipitate extremely fine AIN, using nitrogen permeating from the surface and by carrying out quenching or quenching/tempering, starting from a temperature region where the parent phase is austenite

TECHNICAL FIELD

The present invention relates to high surface pressure resistant steelparts and producing methods thereof. The high surface pressure resistantsteel parts are suitably used as power transmitting parts which arerequired to have contact fatigue strength and wear resistance andexamples of which are rolling members (e.g., gears and bearings), theraces of a rolling member and cam components.

BACKGROUND ART

In recent years, mechanical reduction gears and transmissions areincreasingly required to have high power transmitting capability to meetthe trend toward high output power, light weight and compactness. Morecompactness and higher surface pressure strength are requiredparticularly in gears and bearings.

High contact fatigue strength is also required in gears and bearingsused as power transmitting elements in automotive and constructionmachinery applications. As a measure for enhancing surface pressurestrength, a treatment such as carburization or nitriding is widelyapplied to gears for the purpose of surface hardening. Another measuresuch as addition of Mo to steel is also taken to increase surfacepressure strength, whereby the hardness of the surface as well asresistance to softening caused by tempering is increased. A methodwidely used in recent years is such that a carburization orcarburization/carbonitriding treatment is applied to steel, and then,quenching and shot peening are carried out in order to significantlyincrease surface hardness, while providing considerable compressiveresidual stress to the steel.

There has been reported a method in which a high density cementite phaseis precipitated on the surface of steel through carburization therebyincreasing surface hardness, tempering softening resistance, andtherefore surface pressure strength.

There has also been reported a development of highly clean steel whichis designed to reduce the amount of inclusions with a view to theprevention of destruction due to contact fatigue, which occurs withinclusions as a starting point.

As noted earlier, a method, in which increased surface pressure strengthis achieved by carburization of steel to which Mo (tempering-softeningresistant element) has been added in a larger amount than theconventional steels, is known as an attempt to increase surface hardnessand to restrict a decrease in hardness due to exothermic reaction causedby minute shear deformation resulting from friction and contact stress(Hertzian stress) which occur during rolling movement or rollingmovement accompanied with sliding. In practice, this method, however,presents the following drawbacks: In spite of enhancingtempering-softening resistance, the thickness of an oil film formed onthe contact surface decreases with increases in contact stress,resulting in a significant increase in wear because of the degradationof lubricating properties. This further promotes exothermic reaction andcontact stress, which is a cause of creation of destructive shearingstress. Therefore, the desired, satisfactory improvement in surfacepressure strength cannot be expected. Furthermore, the addition of largeamounts of a tempering-softening resistance enhancing element such as Moentails a considerable increase in the production costs of steelmaterials.

A known method, in which intensive shot peening is applied to thesurface of a carburized article to allow the martensitic transformationof residual austenite which exists in the region extending from theoutermost surface to a depth of about 200 μm below the surface so thathigher surface hardness and greater compressive residual stress areachieved, thereby improving surface pressure strength, does notnecessarily have versatility for the following reasons. Microscopicdefects are created by shots to a grain boundary oxidation layer(defective layer) which has been created during carburization. When thesteel article is in the initial stage of rolling operation, thesedefects bring about wear chip powder generation and surface roughness,resulting in an increase in wear factors. Another reason is that, in thecase of gears, a chip in a tooth attributable to the buildup of strongresidual stress as well as the presence of compressive residual stressadversely affects spalling resistance and, therefore, surface roughnesscauses an increase in wear factors, which results in a decrease insurface pressure strength on the contrary.

There is a case where a gear is subjected to high-carbon carburizationor high concentration carburization in which a high density of cementitephase is precipitated on the surface layer of the gear by carburizing ina different manner. There is also a case where the hardness of a bearingsurface is increased basically by the effect of cementite precipitationsimilarly to the case of bearing steel such as SUJ2 in which cementiteis granulated and finely dispersed, while tempering-softening resistanceis improved by the effect of particle dispersion. However, where a highdensity of cementite is precipitated by the above high carboncarburization process, the precipitated cementite is large in size,namely 5 to 10 μm, so that the agglomeration of cementite is likely tooccur and an extremely large scale of precipitation appears along grainboundaries. As a result, the agglomerated cementite is destroyed by ashearing force generated from contact stress, forming starting pointsfrom which surface defects will occur. If this method is applied tomanufacture of gears, the strength of the dedendum will be decreased.

Attempts have been made to fine cementite and prevent the cementiteagglomeration by an improved high carbon carburization process or by achoice of adequate alloy elements for use in steel. For instance,Japanese Patent Publication (KOKAI) Gazette No. 4-160135 (1992)discloses a method according to which the concentration of Cr isincreased to 2 to 8 wt %, one or more elements selected from the groupconsisting of 0.5 to 4 wt % Ni, 0.01 to 0.5 wt % Nb, 0.1 to 2 wt % V,and 0.05 to 1 wt % Mo are added, and the surface carbon content aftercarburization is increased to 2.0 wt % or more, whereby the carbides andcarbonitrides of V and Cr of 5 μm or less are precipitated in the regionextending from the surface to a depth of 150 μm below the surface. Thismethod is, however, costly, because of the addition of large amounts ofCr for the purpose of facilitating cementite precipitation during acarburizing phase and the addition of V for the purpose of restrainingthe agglomeration/grow of precipitated cementite. Additionally, theconcentration of Cr, V, Mo, Mn and the like in the precipitatedcementite causes a decrease in the concentration of these alloy elementsin the parent phase of austenite, which leads to formation of animperfect quenched layer due to a lack of quenching ability aftercarburization. In order to prevent the formation of an imperfectquenched layer, Ni, which hardly concentrates in a carbide, is addedand, in consequence, the material becomes more expensive.

Japanese Patent Publication (KOKAI) Gazette No. 8-120438 (1996)discloses a method and material for restraining formation of animperfect quenched layer in a quenching process while employing lessexpensive alloy designs. In this publication, surface carbon content isestablished at 1.5 wt % or less in order to prevent the growth andagglomeration of precipitated carbides having grain size exceeding 5 μm.Since the optimum carbon content is 1.5 wt % or less, the amount ofprecipitated carbides is rather small, that is, approximately 7% byvolume or less. In addition, permeating nitrogen is not effectivelyutilized in precipitating carbides or carbonitrides but is mostlydissolved in the parent phase of austenite to be utilized only forpreventing formation of an imperfect quenched layer during quenching.

The method disclosed in the publication 8-120438, however, reveals thefollowing disadvantages. The method of the publication does notprecipitate a large amount of a carbide (cementite) like theconventional high carbon carburization process. Therefore, this methodcannot be expected to have improved surface pressure strength which isattributable to improvements in hardness and in tempering-softeningresistance owing to a large amount of cementite precipitation. In otherwords, the formation of an imperfect quenched layer, which is due to theconcentration of a large amount of quench promoting elements such as Crand Mo in cementite and a lack of these elements concentrated in theparent phase of austenite, is prevented at the cost of a small amount ofcementite precipitation. With this arrangement, the method intends tomake a balance between surface pressure strength and rotary bendingfatigue strength, but in reality, it fails to ensure satisfactorysurface pressure strength. A method similar to the method of the abovepublication is disclosed in Japanese Patent Publication (KOKAI) GazetteNo. 8-3720. According to No. 8-3720, a large amount of cementite isprecipitated, while Ni which is likely to exclude cementite and promotesquenching and Mo which does not concentrate to a large extent incementite and effectively promotes quenching are added in large amountsin order to ensure quenching properties.

In manufacture of shaft supporting products such as bearings, highlyclean bearing steel is used in many cases in order to ensure longrolling life. To produce such clean steel, a steel material undergoessufficient degassing at the steel refining stage and undergoes manystages of desulfurization and dephosphorization with special slug,thereby reducing inclusions such as oxides, nitrides and sulfides. Ithas been reported that rolling life can be increased by approximately 10times by use of a highly clean bearing steel in which the quantity ofinclusions such as oxides and sulfides of 10 to 20 μm is reduced.Low-carbon steel for machine structural use, which is generally used inproduction of ordinary gears, does not ensure sufficient cleanness.Moreover, even if highly clean steel for machine structural use could beproduced, its production cost would be extremely high. Therefore, thereremains a need for the development of an economical technique which iscapable of improving surface pressure strength, even if a steel materialhaving the same inclusion level as that of currently manufactured steelfor machine structural use is used.

Even if the quantity of inclusions contained in a steel can be reduced,the steel is liable to damage and fatigue starting from its contactsurface, because of dust included in a lubricating oil, wear powder, andthe like. For producing high surface pressure resistant parts, it isnecessary to incorporate a surface reinforcement technique to withstandsuch contamination.

The present invention has been directed to overcoming the foregoingproblems and the primary object of the invention is therefore to providehigh surface pressure resistant steel parts and their producing methods,the steel parts meeting the trend toward high strength and compactnessand being suited for use as gears, cams and bearings which are requiredto have wear resistance and strength for withstanding the workingconditions of rolling movement and rolling movement accompanied withslipping.

DISCLOSURE OF THE INVENTION

The above object can be accomplished by a high surface pressureresistant steel part according to the invention. This steel partcontains a nitride which is dispersed in a surface structure and has anaverage grain size of 3 μm or less, and has a multi phase structurecomposed of martensite as a parent phase which is finely divided intopieces by the dispersed substance.

According to the invention, there is provided a method of producing ahigh surface pressure resistant steel part, the method comprising thesteps of:

precipitating a nitride while allowing carbon and nitrogen to diffuselypermeate from a surface by carbonitriding and/orcarburization/(carbonitriding; and

forming a martensite phase subsequently to or independently of thenitride precipitating step, by rapidly cooling from the austenitetemperature region of steel.

One of the features of the invention resides in that a fine nitrideand/or fine carbonitride having at least an average grain size of 0.3 μmor less is essentially dispersed in a contact surface structure, so thata multi phase structure is formed which comprises disordered martensiteas a parent phase, the martensite being divided into much finer piecesby the nitride and/or carbonitride, compared to the conventionallenticular martensite. In addition, the invention uses a steel in whicha carbide (cementite) of 3 μm or less is dispersed thereby reinforcingits surface, 20 to 80% by volume residual austenite is added as astructural component thereby improving toughness, and at least 0.3 to3.0 wt % Al is contained thereby improving resistance to dust containedin a lubricant and to inclusions contained in the steel.

The invention is also associated with a method of producing a highsurface pressure resistant part through the steps of: applyingcarburization, carbonitriding or carburization/carbonitriding to a steelmaterial to cause precipitation of extremely fine AIN in an amount of0.5 to 4.5 wt % (when this amount is represented on the basis ofpercentage by volume, the figure is about 2.4 times the figurerepresented by percentage by weight) by use of nitrogen which ispermeated from the surface; precipitating cementite having an averagegrain size of 3 μm or less in an amount up to 30% by volume; andstarting quenching or quenching/tempering from the range of temperatureat which the parent phase is austenite. It should be noted that thepenetration depth of nitrogen should be arbitrarily adjusted accordingto the distance from the surface to the position where the maximumshearing stress occurs. The maximum shearing stress is generated by themaximum contact pressure (i.e., Hertz's contact pressure) exerted on therolling contact surface of an article to be produced. Generally, thepenetration depth of nitrogen is 1 mm or less and more preferably 0.5 mmor less in view of economical carburization/carbonitriding andeconomical carbonitriding.

As noted earlier, most of the conventional techniques aim at highhardness and improved tempering-softening resistance. In terms ofstructure, they are intended for prevention of an occurrence of surfacecracking caused by contact stress. Therefore, there still remains thecommon problem that surface cracking once occurred cannot be preventedfrom spreading.

The invention is not only intended to improve the prior art in terms ofprevention of an occurrence of surface cracking but also givesconsideration, from the viewpoint of histology, to a vital action forretarding a spread of cracking after it has occurred at the surface.After observation of the spreading route of cracking, it has been foundthat cracking spreading along lenticular martensite or lath martensiteis an important factor for the development of a reinforcement mechanism,apart from cracking spreading along the defects of a material (thedefects include: inclusions and carbide-aggregations contained in amaterial; a grain boundary oxidation layer formed at the stage ofcarburization or carburization/carbonitriding; large cementite particlesand their aggregations; an imperfect quenched layer formed at the stageof quenching; previous austenite grain boundaries caused by segregationor quenching cracks). A lenticular martensite particle in an ordinarycarburized and quenched structure is comparatively large, that is, abouta few μm to tens of μm, and residual austenite particles of low hardnessare present around a martensite particle, causing a situation in which astress is likely to concentrate. Therefore, fining of the martensite isconsidered to be important. However, even if austenite crystal grainsare fined with the intention of fining martensite (which is a measureusually taken), the size of the martensite after fining is limited toabout a few μm, and thus, this measure has been found to beunsatisfactory as an improved reinforcement measure. According to theinvention, 1 to 15% by volume of a nitride and/or carbonitride, whichtakes the form of spheres, rods or needles and has an average grain sizeof 0.3 μm or less, is so dispersed in high concentration as to penetrateinto the martensite particles whereby the width of a martensite particlecan be considerably reduced to about 1 μm or less. By virtue of thepresence of a nitride and/or carbonitride serving as an obstacle, themartensite particles are bent or curved, structurally changing from asimple lenticular configuration to a fine disordered configuration. Theinvention thus exerts resistance to a spread of cracking. The abovearrangement also has an effect on fine dispersion of the residualaustenite distributed around the martensite particles. Further, when apart of the residual austenite undergoes martensite transformation owingto contact stress, it is formed into extremely fine, disorderedmartensite. It is conceivable that such transformation has the effect ofcausing a substantial decrease in stress concentration which occurs whenlarge lenticular martensite particles collide with one another. Suchtransformation also mitigates the concentration of tensile residualstress in the residual austenite existing around the lenticularmartensite, which, conceivably, has the effect of preventing a spread ofcracking along the martensite.

As the dispersed substance, a precipitated substance much finer than thepreviously disclosed cementite is necessary (it should be noted, asshown in the photograph shown in FIG. 7 (described later), fining ofmartensite is hardly achieved by cementite precipitation). Therefore,the invention is arranged to chiefly precipitate a fine Al nitridethrough carbonitriding. The Al nitride has the followingcharacteristics. It has markedly little solid solubility with respect tothe parent phase of austenite. The Al nitride is negatively larger thancementite in free energy for forming a dispersed substance from theparent phase (this means that the Al nitride can be more stablyprecipitated than cementite). Additionally, it does not react withcarbon originally contained in the steel, forming a carbide. It hasprecipitating speed much slower than the growing speed of cementitewhich starts to grow at the same time that Al nitride precipitationstarts. The precipitation of the Al nitride is not a factor ofsubstantially impairing the quenching characteristics of steel.

The contact surface of a part subjected to rolling movement accompaniedwith sliding is damaged, not only by cracks on the surface and in itsvicinity, but also by wear and seizure at the outermost surface. It ispossible to combine the known technique of the prior art with theabove-described arrangement of the invention for improving surfacestrength and wear resistance. For instance, cementite of a few μm may bedispersed in high concentration by carburization to ensure high surfacehardness.

The above-described carburization/carbonitriding treatment is performedin cases where the carbon concentration of a steel at an early stage islow like case hardening steel. On the other hand, in the case ofhigh-carbon steel having carbon content as high as bearing steel,satisfactory improvement in surface pressure strength can be assured byprecipitating a nitride only by carbonitriding at 800 to 850° C. in anatmosphere in which decarburization does not occur. It will beunderstood that although a bearing steel such as SUJ2 has a structure inwhich granulated cementite having an average grain size of 0.3 to 1.0 μmis dispersed in an amount of about 1 to 2.5% by volume, theconfiguration of the martensite in the quenched structure is lenticularand therefore the improvement such as achieved by the martensiteconfiguration of the invention cannot be obtained with the above amountand size of granulated cementite.

In view of the fact that the structural transformation described in thepreviously disclosed technique (Japanese Patent Publication (KOKAI) No.8-120438) is substantially equivalent to that of a bearing steel such asSUJ2 in terms of the size and amount of precipitated cementite, it isconceivable that the martensite parent phase of this publication islenticular martensite. The steel disclosed in the publication No.8-120438 differs from bearing steel in the addition of V, but V does nothave a strong influence on the configuration of martensite parent phasefor the following reason. V can be dissolved in an amount up to about0.28 wt % at a carburized surface at a temperature of 930° C.(carburizing temperature). VC does not precipitate in a substantialamount but most of V is dissolved in cementite. In a case where V isadded in an amount of 1 wt % which is the maximum amount disclosed inthe publication, about 0.5 wt % V is already precipitated as a VCspecial carbide in the steel material, 0.28 wt % V is dissolved inaustenite, and the remaining amount (i.e., 0.22 wt %) of V is expectedto precipitate as a fine carbide during the carburizing process. Theprecipitating amount of a VC carbide is about 0.25% by volume which isvery small compared to the precipitating amount of cementite disclosedin the embodiment of the invention.

As a measure of lessening the stress concentration occurring when theinclusions of the steel or dust included in a lubricant is entangled,the known technique of adjusting the amount of residual austenite iscombined with the above-described technique of the invention. In view ofthe fact that the roller pitting life of the steel drops, when shotpeening is applied to the steel of the invention as described later,after carburization/carbonitriding and quenching so that the amount ofresidual austenite in the surface layer is reduced to 10 to 15% byvolume, the amount of residual austenite is established at 20% by volumeor more and the upper limit of residual austenite is generallyestablished at 80% by volume. If the amount of residual austenite is 80%by volume or more, wear resistance will decrease. It should be notedthat the preferable amount of residual austenite is 20 to 60% by volume.

The quantitative control for residual austenite is carried out bycontrolling carbon potential and nitrogen potential at the time ofcarburization/carbonitriding or carbonitriding. In addition to thequantitative control, a mechanical pressurizing treatment such as shotsor rolling or a thermal treatment such as subzero treatment is appliedto the surface to transform the residual austenite phase to martensite,and a final adjustment is made from the viewpoint of the optimization ofthe surface hardness of steel.

To attain a fine nitride and/or carbonitride within the above multiphase structure, the amount of Al is established at 0.3 to 3.0 wt % andthe amount of nitrogen contained in the surface is generally establishedat 0.4 to 2.5 wt %. When taking into account the relationship with theamount of Al (described later), the preferable amount of nitrogen is 0.7to 1.7 wt %.

It has been confirmed that with Al in amounts of 0.2 wt % or more, theabove-described fining effect on martensite as well as an improvement insurface pressure strength can be achieved. The upper limit of the amountof Al is not strictly limited, but is generally 3 wt % or more andpreferably 0.5 to 2 wt % in view of the fact that addition of Al inamounts of 4 wt % or more causes precipitation of a ferrite phase in theinternal structure of the steel of the high surface pressure resistantpart and impairs the processability of the material.

Although the nitride dispersing effect observed in addition of Al may beexpected by addition of V, the substantial amount of V effective innitride dispersion is 0.2 wt % or less, which cannot achieve the sameeffect as obtained by addition of Al. The steel of the invention may bearranged to contain a carbide (cementite) precipitated on its surface inan amount up to about 30% by volume in order to further improve wearresistance and tempering-softening resistance, and arranged to contain0.5 to 5.0 wt % Cr and/or 0.2 to 10 wt % V for the purpose of adjustingthe average grain size of the precipitated cementite to 3 μm or lessthereby preventing the decrease of fatigue strength due to thedispersion of the carbide. It is more preferable to employ thesearrangements in combination with addition of Al.

In cases where the main purpose of addition of V is not precipitation ofa nitride but fining of the cementite present on the surface, the upperlimit of V is a few wt % on assumption that high temperaturecarburization at 1,100° C. (this is normal temperature) is carried out.Taking the cost into account, the upper limit of V is established at 2wt % in the invention.

It is known that an Al nitride and/or Al carbonitride is most finelyprecipitated on the outermost surface and its grain size increasessubstantially in proportion to the depth of a region from the surface.In practice, when permeating nitrogen at a temperature of 900° C. orless, the average grain size is 0.3 μm or less in the region extendingfrom the surface to a depth of 0.5 mm below the surface. Generally,shearing stress generated by contact stress is maximum in the regionextending from the contact surface to a depth of 0.5 mm below thesurface in many cases and therefore there is no problem. For finelydispersing the precipitated Al nitride and/or Al carbonitride, it isimportant to set a low temperature for the stage of nitrogen permeation.To ensure the quenching characteristics of the steel, the temperature atwhich nitrogen is permeated is preferably 800 to 850° C.

It will be understood from the above description thattempering-softening resistance can be remarkably improved by martensitein which an extremely fine Al nitride and/or Al carbonitride is denselydispersed and the improvement of tempering-softening resistance leads toan improvement in surface pressure strength.

The martensite is divided by the fine Al nitride and/or Al carbonitrideso as to have a grain length of about 1 μm in the vicinity of theoutermost surface. The martensite is so fined that its tissue isopt-microscopically indistinct. Such martensite is thought to have asignificant effect of preventing cracking due to fatigue in the surfaceregion.

The carbon concentration of the steel at its surface duringcarburization/carbonitriding and/or carbonitriding is established at atleast 0.6 wt % or more in order to attain surface hardness, while thenitrogen concentration is established at 0.4 wt % or more in order toadjust the amount of residual austenite to 20% by volume or more. Theupper limit of nitrogen content is allowed to vary according to themaximum value of Al concentration and established at 2.5 wt % or less inorder to adjust the amount of residual austenite to its maximum value,that is, 80% by volume. The preferable amount of nitrogen is 0.7 to 1.7wt % when taking the above-noted range of Al content into account.Cementite precipitation increases as carbon content increases aftercarbon content exceeds 1.1 wt %. The average grain size of cementiteshould not exceed 3 μm in order to prevent the agglomeration ofcementite thereby preventing the decrease of roller pitting strength androtary bending fatigue strength, and the amount of cementiteprecipitation with which the cementite grain size does not exceed 3 μmis about 30% by volume. To adjust the amount of cementite precipitationto 30% by volume, the upper limit of carbon content is established at3.0 wt %. It should be noted that if the carbon content is 3.0 wt % ormore, it becomes difficult to prevent the agglomeration of cementiteparticles even by addition of Cr and V and as a result, the decrease ofbending fatigue strength and pitting strength cannot be effectivelyprevented.

For fine cementite precipitation, it is necessary to add Cr in an amountof 0.5 wt % or more. An addition of V in an amount of 0.2 wt % or morein combination with a Cr addition is more effective, because of theinfluence of the alloy element on the grain size of cementiteprecipitating in the austenite parent phase. More specifically, an alloyelement, which is more likely to concentrate in cementite at theprecipitating temperature, makes the cementite finer (i.e., where theconcentration of an alloy element in cementite/the concentration of analloy element in the austenite parent phase=distribution coefficient KM,an alloy element is more likely to concentrate in cementite, making theaverage grain size of the cementite smaller with increases in thedistribution coefficient KM of the alloy element). Of the alloy elementsgenerally used in ordinary steel for machine structural use, Cr and Vhave a large distribution coefficient, and therefore they have thestrong effect of fining cementite. (According to the survey made by theinventors at a temperature of 900° C., the distribution coefficient KCrof Cr is 6.4, the distribution coefficient KV of V is 12.3, thedistribution coefficient KMn of Mn is 2.1, the distribution coefficientKMo of Mo is 3.5, and the distribution coefficient KNi of Ni is 0.22.)Al has little solid solubility with respect to cementite and thereforedoes not effectively work on fining of cementite. However, Al exerts afining effect although it is little, since Al needs to be forciblyexpelled from cementite during the grow of cementite particles.Therefore, in cases where a large amount of Al is added like theinvention, it works effectively. Precipitation of cementite in an amountof 30% by volume allows Al to significantly concentrate in the austeniteparent phase, promoting the reaction of Al and permeating nitrogen toprecipitate an Al nitride and/or Al carbonitride. Accordingly, thesynergistic effects of cementite precipitation and Al precipitation canbe expected. The upper limit of the amount of Cr is 5 wt % or less inview of the balance between quenching characteristics and cost.

The steel for machine structural use employed in the invention containsAl as an essential element and further contains at least one elementselected from V and Cr. To attain strength at the core of the highsurface pressure resistant part, other elements should be added in thefollowing ranges.

C is an essential element for imparting the desired strength to thecore. Generally, steel material for gears needs to have a carbonconcentration of about 0.1 to 0.5 wt %. Bearing steel, to whichcarburization treatment is not applied when it is a finished product,generally contains carbon in an amount up to 1.2 wt % and undergoes thespheroidizing treatment. If carbon content exceeds 1.2 wt %, theagglomeration of cementite is promoted by spheroidizing of cementite, sothat the desired rolling life cannot be expected. For the above reasons,the carbon contents of the steel material used in the invention ispreferably established at 0.1 to 1.2 wt %.

Generally, Si is inevitably used as an ordinary element in an amount of0.2 wt %. Si is also used conventionally as an element for enhancingtempering-softening resistance, and in such a case, the amount of Si is1 wt % or less. Si can be expected to provide a martensite fining effectin carbonitriding like Al, but Si promotes grain boundary oxidation andcauses variations in carburizing characteristics. Therefore, the amountof Si is preferably limited to 1 wt % or less.

Mn, Ni and Mo play an important part respectively in the quenchingcharacteristics obtained after carburization/carbonitriding and/orcarbonitriding. It is preferred for the invention to use them in amountswithin the ranges which are usually adapted in steel for machinestructural use. (For instance, Mn=0.1 to 1.5 wt %; N=0 to 4 wt %; Mo=0to 1.0 wt %; trace amounts of boron)

Nb, Ti and Zr are added in slight amounts for the purpose of fining thecrystal grains of steel material. In the invention, it is preferred touse them within the range usually adapted. They are also expected tofunction as a nitride-forming element like Al, contributing toprecipitation of a fine nitride. However, they are highly reactive tocarbon, when contained in steel material so that they precipitate inmost amounts prior to carburization/carbonitriding and/orcarbonitriding. Therefore, they need to be added in large amounts inorder to precipitate a nitride, reacting with permeating nitrogen, whichcosts very high. In the invention, taking the above facts into account,their use is limited to fining the crystal grains of steel and theamounts of them are therefore limited to the range of from 0 to 0.1 wt%.

Ca, S and Pb are usually added for the main purpose of improvingmachinability. To achieve improved machinability, the amounts of theseelements are preferably adjusted according to their purposes of use inview of the object of the invention, that is, improved surface pressurestrength.

As described above, the surface carbon content and surface nitrogencontent of steel are controlled by carburization/carbonitriding and/orcarbonitriding, thereby positively adjusting the amount of residualaustenite remaining after quenching in a region close to the surfacelayer. It is also possible to adjust the amount of residual austenite bya physical or thermal means such as the above-noted shot peening orsubzero treatment. The rolling life of steel having residual austenite,the amount of which has been reduced to 10% by volume or less by shotpeening, tends to vary significantly (the amount of residual austenitewas obtained by the X-ray analysis conducted from the surface layer). Itwas also observed that the surface pressure strength of steel tends tobe stable by the presence of residual austenite in an amount of 20% byvolume or more. If 80% by volume of residual austenite is present, wearin the rolling surface progresses at such a rate that the rollingsurface is worn out earlier than the rolling life and the surfacehardness of the steel considerably decreases. Therefore, the preferableamount of residual austenite is 20 to 60% by volume in the regionextending from the outermost surface to a depth of 50 μm below thesurface and 20 to 80% by volume in the region extending from the surfaceto a depth of 0.5 mm.

The carburization, carburization/carbonitriding and/or carbonitridingtreatment of the invention is carried out in the following way. Aftercarburization is once performed at 900° C. or more, temperature isdecreased to about 850° C. Then, a carbonitriding atmosphere isestablished with ammonia gas being additionally introduced andcarbonitriding is carried out without decarburization or bycarburization. An alternative treatment is as follows. Aftercarburization has been carried out at a high temperature ranging from930° C. to 1,100° C. to obtain surface carbon content within the rangeof from 1.1 wt % to about 2 wt %, temperature is once dropped toA₁-point temperature or less and the structure is changed to bainite,martensite or pearite. Then, the steel is reheated to A₁-pointtemperature or more, and while fine granulate cementite beingprecipitated or dispersed at temperature of 900° C. or less, carbonand/or nitrogen is diffusely permeated in thecarburization/carbonitriding atmosphere or in the carbonitridingatmosphere. With this process, the carbon content of the resultant steelis made to be up to 3.0 wt % and the nitrogen content is made to be upto 2.5 wt %, with which the amount of cementite does not exceed 30% byvolume.

The carburization/carbonitriding and/or carbonitriding of the inventionis not limited to a particular method. The ordinary gascariburization/carbonitriding or gas carbonitriding treatment may beadapted. Alternatively, carburization/carbonitriding or carbonitridingcarried out under a reduced gas atmosphere or plasma atmosphere may beemployed. In any cases, processing time may be adjusted so as to meetthe above-described carburization/carbonitriding or carbonitridingconditions.

In the invention, extremely fine nitrides are densely, dispersedlyprecipitated in the rolling surface layer of a gear or bearing in thecarburization/carbonitriding and/or carbonitriding process byeffectively adding nitride-forming elements such as Al and V. In thesubsequent quenching process, disordered acicular martensite finelydivided by the precipitated nitride or carbonitride is formed in theparent phase in order to restrict the occurrence and spread of crackingdue to fatigue in the vicinity of the surface when the resultant articleis in rolling operation. Accordingly, the surface pressure strength ofthe part for use in high surface pressure applications can bedramatically increased.

In addition, a nitride and/or carbonitride containing Al as a chiefcomponent is used as the dispersed substance for effectively dividingand fining the lenticular martensite. By the use of Al, the precipitatecan be extremely fined. Al is not reactive to carbon originallycontained in the steel and substantially all of its amount can beeffectively used in forming the precipitate in the reaction during thecarburization/carbonitriding and/or carbonitriding process. In additionto the dispersion effect of the Al nitride and/or Al carbonitride,surface pressure strength can be increasingly improved while preventingthe decrease of fatigue strength, by adding Cr and V to steel materialto dispersedly precipitate a large amount of fine cementite having asize of 3 μm or less at the surface of the material.

After the above-described carburization/carbonitriding and/orcarbonitriding process, oil quenching or water quenching is carried outto harden the carburization-carbonitrided/carbonitrided part so that ahard layer, in which martensite is fined by an extremely fine,precipitated Al nitride and/or Al carbonitride, can be formed and theresultant part has superior pitting resistance. Quenching preferablystarts from a temperature equal to or higher than the A₁ pointtransition temperature of steel. Alternatively, quenching is carried outby reheating to a temperature equal to or more than the A₁ pointtemperature after cooling to a temperature lower than the A₁ pointtemperature. In the case of the steel of the invention in which an Alnitride is precipitated, the crystal grains of the carbonitrided layercan be easily fined to have an average grain size of about 5 μm or lessby reheating quenching, and this contributes to an improvement in rotarybending fatigue strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a specimen of a high surface pressure resistantsteel part prepared according to one embodiment of the invention, thespecimen being used for rotary bending tests.

FIGS. 2(a) and 2(b) are side views of specimens of a small roller and alarge roller respectively, these specimens being prepared according tothe embodiment and used for roller pitting tests.

FIG. 3 is an explanatory view of a carburization/carbonitridingtreatment according to the embodiment in which no cementite isprecipitated in a carburization phase.

FIG. 4 is an explanatory view of a carburization/carbonitridingtreatment according to the embodiment in which cementite is precipitatedin a carburization phase.

FIG. 5 is a process diagram of a carbonitriding treatment according tothe embodiment.

FIG. 6 shows the results of tests for checking the pitting life of smallrollers after undergoing the carburization/carbonitriding treatmentaccording to the embodiment in which no cementite is precipitated in thecarburization phase.

FIG. 7 is photographs which show, for comparison, the metallographicstructures of Specimen No. 6 (prepared according to the embodiment) andSpecimen No. 11, at regions in the vicinity of their respectivesurfaces.

FIG. 8 is high magnification photographs showing the metallographicstructures of Specimens Nos. 6 and 11 at the regions extending fromtheir respective outermost surfaces to a depth of 100 μm.

FIG. 9 is photographs of the metallographic structures of Specimens Nos.6 and 11 at the surface, showing the result of a nitrogen analysis byuse of EPMA.

FIG. 10 shows the pitting life of rollers at their carbonitridedsurfaces formed by quenching subsequent to thecarburization/carbonitriding treatment of the embodiment.

FIG. 11 shows the pitting life of small rollers after undergoing thecarburization/carbonitriding treatment of the embodiment in whichcementite is precipitated in the carburization phase.

FIG. 12 shows the rotary bending fatigue strength of the specimens afterundergoing the thermal treatment of the carburization/carbonitridingtreatment of the embodiment in which no cementite is precipitated in thecarburization phase.

FIG. 13 shows the fatigue strength of rotary bending test specimens towhich shot peening is applied subsequently to the thermal treatment ofthe carburization/carbonitriding treatment of the embodiment in which nocementite is precipitated in the carburization phase.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, there will be explained high surfacepressure resistant steel parts and methods for producing thereofaccording to preferred embodiments of the invention.

TABLE 1 shows the compositions of steels prepared according to theinvention and steels prepared for the purpose of comparison. The carboncontents of these specimens range from 0.2 wt % to 1.1 wt %. A carboncontent of 0.2 wt % is often employed in the manufacture of casehardening steel for gears and the like, while a carbon content of 1.1 wt% is often employed in the manufacture of medium carbon steel and SUJ2that is a representative example of bearing steel. Specimens No. 1 to 8are prepared with the intention of checking the effects of additions ofAl and V. With these specimens, the dispersion effect of an Al nitrideand/or Al carbonitride was checked. Also, the combined effects ofdispersion of fine cementite and dispersion of an Al nitride and/or Alcarbonitride were checked. Specimens Nos. 9 to 12 are medium carbonsteel, SNCM420H, 420H and SUJ2, respectively and prepared as comparativeexamples.

TABLE 1 ALLOY COMPOSITIONS OF SAMPLES STEELS C Si Mn Cr Ni Mo V AlMATERIALS PREPARED ACCORDING TO THE INVENTION No. 1 0.55 0.24 0.75 0.470.52 No. 2 0.2 0.22 0.75 1.02 0.15 0.42 0.031 No. 3 0.21 0.07 1.04 2.850.2 0.03 No. 4 0.22 0.08 1.01 2.81 0.2 1.01 No. 5 0.23 0.21 0.53 1.010.2 0.31 No. 6 0.21 0.22 0.51 1.01 0.21 1.02 No. 7 0.35 0.09 1.25 1 0.192.53 No. 8 1.04 0.27 0.48 1.51 0.75 MATERIALS PREPARED FOR COMPARISONNo. 9 0.56 0.25 0.76 0.51 0.031 No. 0.21 0.28 0.52 0.62 1.89 0.22 0.05Nb10 No. 0.22 0.26 0.76 1.03 0.19 0.029 11 No. 1.02 0.28 0.5 1.47 12

After casting, these sample steels were subjected to hot forging andnormalizing and then formed into specimens for rotary bending tests andsmall roller specimens for roller pitting tests, as shown in FIGS. 1 and2. For preparing large rollers for roller pitting tests, SUJ2 wasquenched and tempered so as to have a hardness of H_(R) C64. In thepresent embodiment, carburization/carbonitriding and carbonitridingtreatments are carried out through the processes shown in FIGS. 3, 4 and5. During the carburization/carbonitriding and carbonitriding treatment,carbon potential Cp is adjusted by controlling the CO₂ gas concentrationof carburization gas and nitrogen potential Np is adjusted bycontrolling the flow rate of ammonia.

FIG. 3 shows a carburization/carbonitriding treatment in whichcarburization is carried out at 930° C. while adjusting Cp to 0.8 wt %,and then, carbonitriding is carried out at 850° C. This treatment isarranged so as not to precipitate cementite during the carburizationphase and so as to precipitate Al nitrides during the carbonitridingphase. However, when using steels containing Cr in amounts of 1 wt % ormore, there are some cases where a small amount of cementiteprecipitate. The reason for this is that when controlling Cp, thedifficulty of controlling the percentage (0.25%) of CO₂ in RX gas(carburization gas) is avoided. But, such cementite precipitation has noproblem in achieving the substantial effects of Al additions.

FIG. 4 shows a process in which Cp is set to 1.4 wt % and precipitationof cementite is positively promoted during the carburization phase andat the same time, precipitation of an Al nitride is promoted bycarbonitriding at 850° C.

FIG. 5 shows a process in which carbonitriding at a temperature of 830°C. is applied to Specimens Nos. 8 and 12 having carbon content as highas high carbon bearing steel. It should be noted that the temperingtreatments applied to the specimens were all carried out at atemperature of 160° C. for two hours.

All the specimens underwent carbonitriding at 850° C. so as to have anitrogen permeating depth of about 0.2 mm.

After the thermal treatment, the carbon and nitrogen concentrations ofthe surface layer of each specimen were obtained by analyzing thesection of the specimen at appropriate time by EPMA (X-ray microanalyzer) which utilized analytical curves. An Al nitride and/or Alcarbonitride which precipitated in the layer close to the surface andthe martensite structure were observed, using an optical microscope anda scanning election microscope properly. The amounts of residualaustenite and residual stress in each specimen at a position close tothe surface layer were measured after applying electrolytic polishing tothe surface.

A roller pitting fatigue test was conducted under the followingconditions. Each small roller specimen shown in FIG. 2 was pressedagainst a large roller made of SUJ2 and the surface pressure strength ofthe small roller specimen was evaluated under the conditions, that is, arevolution speed of 1,050 rpm, a slip rate of 40%, and a surfacepressure appropriately varied within the range of from 250 to 375kg/mm². The pitting life of each small roller was evaluated on the basisof the number of revolutions until one pitting defect is created in thesmall roller. In the roller pitting tests, when no pitting was foundwithin the revolutions of 20-30×10⁶ times, no more revolution wascarried out after that.

The bending fatigue strength of each small roller was evaluated with arevolution speed of 3,600 rpm and the limit of the number of revolutionswas, 20×10⁶ times.

FIG. 6 shows the pitting life of each small roller specimen which wascarburized and carbonitrided under the conditions shown in FIG. 3. Thelower limit of the variation of pitting life is represented by dashedline in FIG. 6 on the basis of the pitting life of Specimen Nos. 9 to 11which contain substantially no Al. It is apparent from Specimen Nos. 1,4, 5, 6, 7 and 8 which contain Al in amounts of 0.3 wt % or more thatpitting life increases as the amount of Al increases. EspeciallySpecimens Nos. 4 and 6, which contain Al in amounts of about 1 wt %,exhibited such high surface pressure strength that no pitting wascreated in most of the samples even under Hertz's contact pressure of375 kg/mm² which is the maximum surface pressure employed in thisembodiment. The significant effect of an addition of Al can be admittedeven in Specimen No. 5 which contains 0.31 wt % Al. Specimen No. 7containing 2.53 wt % Al exhibits a shorter pitting life than SpecimensNos. 4 and 6 for the following reason. While Specimens Nos. 4 and 6 havea nitrogen carbonitriding depth of 0.2 mm, Specimen No. 7 processedunder the conditions shown in FIG. 3 has less permeating depth whichvaries within the range of from 0.05 to 0.12 mm since the carburizingtime for it was insufficient. The pitting life of Specimen No. 7 can beimproved by extending the time of carburization at 850° C. In SpecimenNo. 2, which contains substantially no Al but contains 0.42 wt % V, theeffect of addition of V can be slightly admitted, but this effect is notsignificant compared to the effect of Al addition. It is understood fromthe fact that Specimen No. 3 containing 2.85 wt % Cr was not improved inpitting life that the effect of addition of Cr is not important and Crdoes not react with permeating nitrogen under thecarburizing/carbonitriding conditions of the invention in whichcementite is not actively precipitated.

FIG. 7 shows, for comparison, the metallographic structures of SpecimensNos. 6 and 11 at regions near their respective surfaces. Specimen No. 6is a representative steel containing Al whereas Specimen No. 11 is itscomparative material. As seen from FIG. 7, extremely fine martensite isprecipitated in the region near the outermost surface in Specimen No. 6,and the average length of the martensite particles is about 1 μm orless. The martensite of Specimen No. 6, which is observed as acicularmartensite by an optical microscope, is composed of a linkage of suchextremely fine martensite particles and accordingly, distinctly differsfrom its comparative material in the structural configuration ofmartensite. FIG. 8 shows high-magnification photographs showing themetallographic structures of Specimens Nos. 6 and 11 at the regionsextending from the respective outermost surfaces to a depth of 100 μmbelow the surfaces. It is seen from FIG. 8 that the martensite ofSpecimen No. 6 includes uniformly dispersed an Al nitride having anaverage grain size of 0.3 μm or less and the martensite is finelydivided into pieces having a width of about 1 μm or less by the fine Alnitride and/or Al carbonitride, forming a disordered martensiteconfiguration. Although a small amount of cementite having a grain sizeof about 0.1 to 0.2 μm is precipitated in the comparative material ofSpecimen No. 11, the martensitic structure of this material issubstantially lenticular martensite having significant linearity.Obviously, the dispersion of cementite having the above size level doesnot have the effect of fining martensite. It is observed in Specimen No.6 that, apart from fine granular ones, a considerable amount of Alnitride and/or Al carbonitride particles having a shape of fiber (rod)is precipitated on the surface and the diameter of these nitrides and/orcarbonitride particles is 0.3 μm or less. One of the features of theinvention resides in such a short fiber-like precipitation form whichhas strengthening effects.

FIG. 9 is photographs of the metallographic structures of the surfacesof Specimens Nos. 6 and 11, showing the result of a nitrogen analysis byuse of EPMA. The nitrogen concentrations of Specimens Nos. 6 and 11 are1.2 wt % and 0.7 wt %, respectively. Although the surface nitrogenconcentrations of the specimens largely vary according to Alconcentrations, the amount of nitrogen changed to AIN is substantiallyequal to the amount of nitrogen calculated from the stoichiometriccomposition of AIN, on assumption that Al which has been added is allchanged to AIN. Specimen No. 6 contains the desired amount of carbon.

It has been confirmed by the X-ray analysis that the amounts of residualaustenite at the respective surface layers of Specimens Nos. 6 and 11substantially fall in the range of from 40 to 60% by volume. Regardingthe amount of residual austenite in the region extending from theoutermost surface to a depth of 20 μm, Specimen No. 6 containing Alsurpasses Specimen No. 11 by an amount of about 12% by volume. As awhole, the amount of residual austenite in a structure quenched afterthe carburization/carbonitriding process is highly dependent on theamount of permeated nitrogen, and it is confirmed that all of thespecimens prepared according to the embodiment contain nitrogen inamounts of 30% by volume or more.

To observe the influence of a carbonitrided surface portion created byquenching after the carburization/carbonitriding process on the rollerpitting life of steel, the region extending from the surface to a depthof 0.10 mm and the region extending from the surface to a depth of 0.15mm were respectively removed from the carbonitrided portion (0.2 mm) ofSpecimen No. 6 which had undergone the carburization/carbonitridingprocess, and pitting was checked. FIG. 10 shows the pitting result.Further, shot peening was applied to the surface layer of Specimen No. 6with an arc height of 0.9, thereby reducing the amount of residualaustenite at the surface to about 14%. The pitting result of this caseis also shown in FIG. 10.

It has been found that as the amount of removal of the carbonitridedlayer increased, pitting life markedly decreased, and finally, thepitting strength of Specimen No. 6 became substantially equal to that ofthe comparative material which had undergone thecarburization/carbonitriding process in which the Al nitride having theeffect of fining martensite was not precipitated. It has been also foundthat the reduction in the amount of residual austenite by shot peeningdecreases the pitting life of, particularly, a steel subjected to themaximum surface pressure (i.e., 375 kg/mm²) of the present embodiment,but the adverse effect of shot peening is not as significant as theadverse effect of the removal of the carbonitrided layer. It is wellknown that shot peening has the good effect of markedly hardening thesurface of steel and creating high compressive residual stress and isalso expected to increase pitting strength, wear resistance and bendingfatigue strength. Hence, it is preferable to utilize shot peening tosuch an extent that the amount of residual austenite at the surface doesnot decrease to about 20% by volume or less.

FIG. 11 shows the pitting life of small roller specimens Nos. 2, 3, 4, 6and 11 which underwent the carburization/carbonitriding process in whichcementite was positively precipitated by permeating a high density ofcarbon under the conditions shown in FIG. 4. FIG. 11 also shows thepitting life of Specimens Nos. 8 and 12 of bearing steel materialshaving a high density of carbon and cementite which is originallyprecipitated therein. These specimens underwent the carbonitridingprocess shown in FIG. 5. The pitting life lower limit line indicated bydashed line in FIG. 6 is also shown as a reference for pitting lifeevaluation in FIG. 11.

It has been found that, generally, the pitting life of the comparativematerials can be improved to a considerable extent by precipitation ofcementite having a grain size of 5 μm or less, and a further improvementin pitting life can be achieved by addition of Al. The effects ofcementite precipitation and Al nitride precipitation for finingmartensite can be distinctly confirmed by, for instance, the comparisonsbetween the life of Specimens Nos. 3 and 4 and between the life ofSpecimens Nos. 8 and 12 having the level of bearing steel. Hence, thecombined effects of the above two kinds of precipitation carried out inthe invention have been confirmed.

Specimens Nos. 2, 6 and 11 have surface carbon contents ranging from 1.5to 1.8 wt % and a structure where cementite having an average grain sizeof 2 to 5 μm is precipitated. Specimens Nos. 3 and 4 have surface carboncontents ranging from 2.3 to 2.6 wt % and a structure where cementitehaving an average grain size of 1.5 to 2.5 μm is precipitated.

Cementite in Specimen No. 11 (SCM420H comparative material) has a grainsize of 5 μm and cementite in Specimen No. 3 has a grain size of 2.5 μm.As the amount of Cr increases, the grain size of the precipitatedcementite decreases, which is consistent with the previous report. Finercementite was observed in steels to which Al was added. It will beunderstood from the relationship between carbon content and the grainsize of cementite and from the structural effects of Al nitrideprecipitation that the above-described effects in improving pitting lifeare substantially rational.

FIG. 12 shows the rotary bending fatigue strengths of Specimens Nos. 6and 11 after applying the thermal treatment shown in FIG. 3. Since anprecipitated Al nitride layer caused by carbonitriding is present in thesurface layer (0.2 mm) of Specimen No. 6, there may be concern aboutstress concentration due to the notching effect of bending stresspresent at the surface. However, it is understood from the results shownin FIG. 12 that there is no need to worry about such stressconcentration. This is because the precipitated Al nitride is extremelyfine, having a grain size of 0.3 μm or less. FIG. 12 also shows therotary bending fatigue strengths of steels to which shot peening wasapplied with an arc height of 0.9 and the substantially same effect ofSpecimen No. 11 can be observed from it. As seen from this result, Alnitride precipitation does not create stress concentration points evenwhen a reinforcement treatment is applied to the surface. The aboveeffect can be also seen in the comparison of the rotary bending fatiguestrengths of Specimens Nos. 8 and 11 (bearing steel) to which thethermal treatment shown in FIG. 5 was applied.

FIG. 13 shows a comparison of the rotary bending fatigue strengths ofSpecimens Nos. 3, 4, 6 and 11 to which the thermal treatment of FIG. 4was applied. As noted earlier, cementite having an average grain size ofabout 5 μm is precipitated on the surface of Specimen No. 11 while thecementite precipitated in Specimen No. 6 has an average grain size ofabout 2 μm. The rotary bending fatigue strength of Specimen No. 11 isabout 10% lower than those of the steels in which no cementite isprecipitated. Substantially no decrease in strength is seen in SpecimenNo. 6 in which the grain size of cementite is restricted to about 2 μmby an addition of Al. In view of this, it is preferable to apply athermal treatment so as, to restrict the average grain size of cementiteto 3 μm or less when precipitating cementite on the surface. FIG. 13also shows the fatigue strengths of Specimens Nos. 6 and 11 for rotarybending tests, which were subjected to thermal treatment and then toshot peening with an arc height of 0.9. Specimen No. 11 containingcementite large in grain size has not achieved a significant improvementin rotary bending fatigue strength while Specimen No. 6 containing finecementite precipitated therein has attained a significant improvement.As seen from this result, it is important, in view of fatigue strength,to restrict the average grain size of cementite to 3 μm or less, but itis more important to eliminate cementite agglomeration. FIG. 13 furthershows the rotary bending fatigue strengths of Specimens Nos. 3 and 4 towhich the thermal treatment of FIG. 4 was applied. It is seen from thisfigure that even when the amount of cementite in Specimens No. 3 and 4is around 25% by volume, fatigue strength can be prevented fromremarkably decreasing by fining cementite particles so as to have grainsizes of 2.5 μm and 1.5 μm, respectively. Hence, surface pressurestrength can be improved without causing a decrease in strength byfining cementite, on condition that the amount of cementite is up toabout 30% by volume. It should, however, be noted that when allowingcementite precipitation exceeding 30% by volume, a large amount ofcementite is likely to agglomerate, forming coarse cementite particlesso that a significant improvement in surface pressure strength cannot beexpected.

It is conceivable that the grain size of the precipitated Al nitride canbe effectively reduced by setting the temperature of the carbonitridingprocess shown in FIGS. 3 to 5 to a low temperature. To employ a lowcarburizing temperature is also desirable for positively precipitatingcementite by carburization as shown in FIG. 4. Practically, it ispreferable to carry out the carburization/carbonitriding andcarbonitriding treatments at a temperature of 800° C. or more.

According to the invention, after a fine precipitate such as Al nitridesare formed by the carburization/carbonitriding and/or carbonitridingtreatment, martensite created by quenching is extremely fined, wherebyfatigue failure caused by contact stress can be decreased to asignificant extent. One of the features of the invention resides in thatthe above effect can be easily, economically achieved by use of steelscontaining inexpensive alloy elements such as Al which do not cause adecrease in rotary bending strength. According to the invention, in thecase of a steel in which cementite precipitation is allowed, Al can beexpelled from the cementite particles so that the quenching propertiesof the parent phase will not be impaired. In addition, Al functions tofine the precipitating cementite particles and effectively promotes theprecipitation of Al nitrides, during the carbonitriding process. Inconsequence, the average life of the steel parts subjected to rollingfatigue can be remarkably raised.

Hence, the present invention is well suited for use in the manufactureof gears, cams, bearings, and similar high-strength and compact steelarticles which are required to have wear resistance and strength forwithstanding rolling or rolling-slipping fatigue.

What is claimed is:
 1. A high surface pressure resistant steel partcontaining a fine nitride and/or carbonitride which is dispersed only ina surface structure extending from the surface to a depth of 1 mm belowthe surface having an average grain size of 0.3 μm or less in an amountof 1% by volume or more and containing a carbide mainly composed ofcementite having an average grain size of 3 μm or less which isdispersed in an amount up to 30% by volume and having a multi phasestructure composed of martensite as a parent phase which is finelydivided into pieces by said dispersed substance and further containing20 to 80% by volume residual austenite in said multi phase structure andcontaining 1.1 wt. % or more carbon in its surface.
 2. A high surfacepressure resistant steel part according to claim 1, wherein saidmartensite of the parent phase is more acicular than the lenticularmartensite of an ordinary carburized structure, is divided into piecesby a dispersed fine nitride and/or carbonitride having a width of 0.3 μmor less, is mainly composed of fine martensite particles having a widthof 1 μm or less, and is considerably disordered.
 3. A high surfacepressure resistant steel part according to any one of claims 1 or 2,formed as a power transmitting part which is required to have contactfatigue strength and wear resistance.
 4. A high surface pressureresistant steel part according to claim 1, whose surface has a carboncontent of 1.1 to 3.0 wt % and a nitrogen content of 0.4 to 2.5 wt %. 5.A high surface pressure resistant steel part according to claim 1, whichcontains 0.3 to 3.0 wt % Al, and containing 0.4 to 2.5 wt % nitrogen inits surface.
 6. A high surface pressure resistant steel part accordingto claim 1 or 5, which is containing 0.5 to 5.0 wt. % Cr and 0.2 to 2.0wt % V, and containing 1.1 to 3.0 wt % carbon in its surface.
 7. A highsurface pressure resistant steel part according to claim 6, containingimpurities and alloy elements selected from the group consisting of Si,Mn, Mo, Ni, B, S and Pb.
 8. A high surface pressure resistant steel partaccording to claim 7, for use as a power transmitting part which isrequired to have contact fatigue strength and wear resistance.
 9. A highsurface pressure resistant steel part according to claim 8, wherein thepower transmitting part is selected from the group consisting of gears,bearings, the races of a rolling member and cam components.